The production of methane-containing gas streams, such as natural gas, frequently involves the removal of contaminents or components which are not desired in the pipeline product of natural gas production. One such class of components has been referred to as natural gas liquids or NGL. NGL constitutes ethane and higher hydrocarbons which are found in natural gas. Depending upon the current economics and gas dewpoint requirements, it is sometimes desirable to remove the NGL from natural gas. Various distillation methods have been considered for such a separation whereby the cooling of the natural gas accomplishes the separation of the NGL components from the methane components of the natural gas.
An exemplary contaminent which is deemed desirable to remove from natural gas before pipelining is nitrogen. Many production streams of a natural gas include minor but not insignificant levels of nitrogen. In order to meet the specified minimum sales gas heating value or maximum concentration of inert components, this nitrogen is not allowed to remain in the natural gas which is pipelined to market or further transportation. In addition to the naturally occurring levels of nitrogen in some produced natural gas, nitrogen in varying concentrations now occurs in produced natural gas wherein the production of the natural gas associated with oil deposits has been maintained or assisted by nitrogen injection into the strata holding the deposits. Such production techniques are typically referred to as enhanced recovery or non-primary recovery and are more widely utilized as the number of naturally producing natural gas and associated oil deposits decline. Various methods have been utilized for the separation of nitrogen from natural gas. These techniques are typically referred to as nitrogen rejection because the nitrogen is removed as a waste stream or by-product of little or no value, although in some instances recovered nitrogen is reinjected into the strata when it is more economical than the production of fresh nitrogen. The nitrogen rejection is generally effected by the cooling of the nitrogen-containing natural gas and fractionating it in a distillation column.
These procedures for the removal of natural gas liquids or nitrogen usually require the reduction in pressure of the natural gas stream being treated in order to obtain the necessary refrigeration for the separation. Such processes are exemplified by the following prior art.
In U.S. Pat. No. 3,656,312, a process is described wherein a liquefied gas mixture containing methane is separated from ethane and heavier hydrocarbons in a distillation column. The methane recovered in the process is in the liquid state and can be easily pumped to the desired pressure. Separate columns for the separation of ethane from propane, butane and heavier hydrocarbons are contemplated.
U.S. Pat. No. 3,797,261 provides a process for the separtion of natural gas containing nitrogen. The natural gas is separated into a low nitrogen fraction and a high nitrogen fraction. The separation is performed in a distillation column wherein refrigeration is derived from the expansion of the high nitrogen fraction and by the vaporization of a recycle medium in heat exchange relation with the vapor in the column. The high nitrogen fraction, having been expanded to produce necessary refrigeration is removed from the process at atmospheric pressure. The process contemplates a recycle medium which performs additional refrigeration duty for the operation of the column. The methane product is reduced in pressure from approximately 300 psia to 25 psia. This requires the methane product to be recompressed to pipeline pressure of approximately 300 psia. The waste nitrogen from this process is withdrawn from the process at 15 psia or approximately atmospheric pressure. The process requires the reduction in pressure in order to provide the necessary refrigeration for the separation. In the case of natural gas containing greater than 50% nitrogen, the 300 psi feed stream is separated into a nitrogen product which is reduced in pressure to at least 20 psia. The product if desired in liquid form is reduced to 30 psia and if provided in gaseous form is reduced to at least 40 psia. Therefore, in order to recover the variable nitrogen content or the gaseous or liquid methane streams in this process, significant pressure reductions for refrigeration duty are necessary.
U.S. Pat. No. 4,323,380 discloses a process for the separation of nitrogen and ethane from natural gas. The process uses three rectification stages in order to perform the separation. The process is directed to a fixed low content nitrogen feed of natural gas.
Linde Reports on Science and Technology, Vol. 15 (1970) discloses a nitrogen rejection process wherein nitrogen-containing natural gas is introduced into a high pressure column which is operated by a closed circuit methane refrigeration cycle. Again, the process is relevant to a fixed nitrogen content feed gas stream.
The prior art processes generally lack the ability to deliver nitrogen and methane at high pressure in a process which is readily utilizable over a wide range of nitrogen content in the feed. The present invention overcomes these drawbacks as will be more fully described.